This invention relates to electronic instruments for detecting and/or measuring the performance characteristics of systems, subsystems, and components incorporated into telecommunications systems and, more particularly, to electronic instruments for detecting and/or measuring the performance of modulated optical sources employed in optical telecommunications systems. Specifically, one embodiment of the invention provides an apparatus and method for determining the extinction ratio of a digitally modulated optical source, such as a laser transmitter, in an optical telecommunications system. Measurement of parameters for determining extinction ratio in accordance with one embodiment of the invention provides a more accurate determination of extinction ratio than has been available in the past, thereby enabling better design and precise characterization of digital laser transmitters.
Extinction ratio is an important performance characteristic of laser transmitters used in optical telecommunications systems. It is a measure of the amplitude of the digital modulation on the optical carrier and, therefore, affects the power penalty, or distance over which an optical fiber telecommunications system can reliably transmit and receive a signal. It is also a characteristic that is very difficult to accurately determine using known measurement techniques.
Standards for optical telecommunications systems, such as SONET, SDH, and Fibre Channel, specify minimum extinction ratio requirements for laser transmitters. Since extinction ratio is explicitly specified in these standards, it is important that any given laser transmitter, when its performance is measured on different test systems, yields a similar extinction ratio value. Nevertheless, a common problem in the industry today is that a laser transmitter manufacturer can measure a transmitter as within specification on one test system, yet the customer measures it as out of specification on another test system.
Considered in more detail, with reference to FIG. 1, extinction ratio is defined as: EQU ER=P.sub.1 /P.sub.0
where P.sub.1 is the average power (or energy) in a logic one bit and P.sub.0 is the average power (or energy) in a logic zero bit. Extinction ratio is often also specified as a dB value: EQU XR=10 log (ER)
Alternatively, some textbooks define extinction ratio as a percent based on the inverse of the above definition: EQU XT=100/ER
This definition is particularly preferred in Europe. In the following analysis, it is considered that extinction ratio is defined by ER, although the analysis is identical irrespective of which definition is adopted.
While it would seem that an infinite extinction ratio value is best, because this represents maximum signal swing, it is not practically achievable for laser transmitters. When a laser transmitter is biased to the completely "off" state (i.e., when transmitting a logic zero), it suffers from turn-on delays and waveform distortions that can cause transmission errors. Therefore, as shown in FIG. 1, laser transmitters are typically biased so that a small amount of optical power above the dark level is transmitted even when transmitting a logic zero pulse. This reduces extinction ratio, and, consequently, the optimum bias point for the laser transmitter is a compromise between least turn-on delay and best extinction ratio.
In accordance with the present state of the art, extinction ratio is determined using measurements with an oscilloscope after converting from the optical signal to an electrical signal with an optical-to-electrical converter, as shown in FIG. 2. The determination of extinction ratio is typically performed based on an eye diagram, which is a convenient way of displaying all possible sequences of logic ones and zeroes by overlapping them on the display of the oscilloscope, as shown in FIG. 3.
Now, the oscilloscope shown in FIG. 2 does not directly measure the average power in the logic one and zero bits. Instead, these values are approximated by determining the average logic one and logic zero voltages, as shown in FIG. 3. This technique is described, for example, in the current draft of Telecommunications Industries Association (TIA) procedure OFSTP-4. See, Optical Eye Pattern Measurement Procedure, OFSTP-4, TIA/EIA-526-4, Telecommunications Industries Association, Draft Standard Proposal No. 2372, 1993.
Unfortunately, the presently used technique for measuring parameters to determine extinction ratio is subject to severe errors that render it impossible to reliably obtain an accurate extinction ratio value. Some of the sources of error are as follows.
The exact definition of extinction ratio requires that the average optical power in the logic one and logic zero pulses be measured. The oscilloscope measurement technique, however, does not measure the true average powers in the logic one and zero bits. Instead, it measures logic one and zero voltages that are related to the peak (not average) optical power at some instant in time.
More particularly, the logic one and zero voltages measured by the oscilloscope depend on several factors. One factor is whether or not the voltage is averaged over the entire bit period or only over a small fraction of time at the center of the bit period. Another factor is whether the average values are defined as the mean of a histogram, the most prevalent value of a histogram, or some other definition. The measurement result is therefore only an approximation of the true extinction ratio and can have a significant error in some cases.
For example, consider the two waveforms illustrated in FIG. 4. According to the exact definition of extinction ratio, the waveform on the left should result in determination or a lower extinction ratio than the one on the right, because the total energy in the bit is less. Using the oscilloscope measurement technique, however, both waveforms would produce the same measurement, because they both reach the same mean peak value.
Another source of error relates to the measurement bandwidth. The exact values of the average logic one and zero voltages are highly dependent on the bandwidth and frequency response of the measurement system. For this reason, OFSTP-4 recommends the measurement bandwidth to be maintained within .+-.0.3 dB of a certain nominal response. Nevertheless, analysis has shown that frequency response variations within this tolerance can cause the extinction ratio on a signal with nominal 10 dB XR to vary by as much as .+-.0.75 dB.
An additional source of error is oscilloscope accuracy. Surprisingly, the ac accuracy of oscilloscopes is not clearly specified. Instead, it must be inferred from the dc accuracy specification and the frequency response specification of the oscilloscope. Important parameters, such as overshoot and ringing, are also rarely guaranteed. Nevertheless, all of these parameters have significant impact on measurements to determine extinction ratio. Accordingly, measurement of parameters for determination of extinction ratio is one of the most difficult measurements for an oscilloscope to perform accurately. Also, because the measurement depends on unspecified oscilloscope parameters, any resulting measurement cannot be guaranteed as traceable to reference standards.
It would therefore be desirable to provide a more accurate determination of extinction ratio. Such an extinction ratio determination would provide more meaningful information to engineers during design of digital laser transmitters and increase reliability with regard to deployment of such transmitters in optical telecommunications systems.
The problem with the present state of the art technique used to measure parameters to determine extinction ratio is that extinction ratio is defined as the ratio of two average optical powers that cannot be directly measured using an oscilloscope. An improved approach would be one which measures these two powers directly. Since power is one of the most basic measurements and can be traced to fundamental standards, a measurement apparatus and method based directly on measurement of power would be the most accurate and repeatable approach possible in connection with determination of extinction ratio.